Digital printing involves technologies in which a printed image is created directly from digital data, for example using electronic layout and/or desktop publishing programs. Known methods of digital printing include full-color ink-jet, electrophotographic printing, laser photo printing, and thermal transfer printing methods.
Electrophotographic printing techniques involve the formation of a latent image on a photoconductor surface mounted on an imaging plate. The photoconductor is first sensitized to light, usually by charging with a corona discharge, and then exposed to light projected through a positive film of the document to be reproduced, resulting in dissipation of the charge in the areas exposed to light. The latent image is subsequently developed into a full image by the attraction of oppositely charged toner particles to the charge remaining on the unexposed areas. The developed image is transferred from the photoconductor to a rubber offset blanket, from which it is transferred to a substrate, such as paper, plastic or other suitable material, by heat or pressure or a combination of both to produce the printed final image.
The latent image is developed using either a dry toner (a colorant mixed with a powder carrier) or a liquid ink (a suspension of a colorant in a liquid carrier). The toner or ink generally adheres to the substrate surface with little penetration into the substrate. The quality of the final image is largely related to the size of the particles, with higher resolution provided by smaller particles.
Dry toners used in solid electrophotography are fine powders with a relatively narrow particle size distribution that are expelled from fine apertures in an application device. A typical dry toner is predominantly composed of a heat-sensitive polymer (e.g., acrylic, styrene) and a pigment such as carbon black with a solid carrier, typically resin coated iron or steel powders. Variations in particle shape and charge-to-mass ratio as well as dust particles found in the dry ink may cause technical difficulties during the printing process. Larger or irregularly shaped particles can cause blockage while dust particles that are too small to hold a sufficient charge to be controllable adhere to the print head surface.
Liquid inks used in liquid electrophotography are generally comprised of pigment- or dye-based thermoplastic resin particles suspended in a non-conducting liquid carrier, generally a saturated hydrocarbon. Offset-preventing and release-facilitating oil, such as silicone oil, is often used to increase the efficiency of ink transfer from the imaging surface. The liquid ink is electrophotographically charged and brought into contact with the photoconductor surface to develop the latent image. When transferred to an offset blanket and heated, the particles melt and fuse to form a tacky polymer film. When the tacky polymer film comes in contact with a cooler surface, such as a paper substrate, the film hardens and adheres to the substrate and peels away from the blanket, laminating the paper. The ink is deposited onto the substrate essentially dry, and desired print finishing can be performed immediately. Since the ink is transferred essentially completely from the blanket to the substrate, a new layer in a different color can be created for every rotation of the press.